The present invention relates to radiation detectors and x-ray energy spectroscopy.
Elemental compositions of many materials are discoverable by the process of x-ray fluorescence, wherein atoms excited by high energy x-ray absorption re-emit the energy as x-rays having lower energy. The energy distribution of the re-emitted x-rays forms an elemental "fingerprint" that is useful for identifying the composition. X-ray spectroscopy is particularly effective for determining elemental abundances in that the energies of the characteristic x-rays of an element are little changed by the environmental matrix of the element, unlike spectroscopy in the ultraviolet, visible and infra-red spectrums.
The energy of these characteristic x-rays generally increases with increasing atomic number (Z), as does the energy spacing between the characteristic emissions. Higher energy x-rays are generally easier to detect than low energy x-rays in that they are more penetrating and generate a larger signal in an energy sensitive detector. Thus elements of higher Z are easier to isolate and quantify than elements of low Z. For example, silicon (Z=14) is used extensively in a wide variety of applications, including semiconductor fabrication and lubrication. Silicon is also used in thin coatings as a release agent on paper and plastic substrates, giving rise to a need for monitoring the thickness and quality of the coatings. X-ray spectroscopy directed to silicon has had limited success in the prior art because of its low atomic number and because of the likely presence of other common low atomic number elements such as aluminum and calcium that have similar x-ray fingerprints.
The performance of an x-ray spectrometer is most often limited by the performance of the x-ray detector utilized, x-ray detectors of the recent prior art exhibiting tradeoffs between energy resolution, response time, and cooling requirements. Available devices having the highest performance require complex support equipment; thus spectrometers utilizing these devices have large physical size and/or slow response times. High performance spectrometers find application in laboratory analysis of metals, ceramics, pharmaceuticals, and other products wherein sample analysis typically requires extensive preparation and then many minutes to hours of data acquisition. Lower performance devices having less sensitivity and energy resolution find application in industrial process quality control, the industrial units typically operating off-line with sample analysis requiring little preparation and only a few minutes of data acquisition time.
Detectors in common use for x-ray energy spectroscopy include scintillation detectors, gas-proportional detector tubes, lithium-drifted silicon detectors, and large-area PIN photodiodes. Scintillation detectors down-shift the x-ray energy to the optical spectrum, requiring further detection by an optical detector such as a photo-multiplier tube, whereas the remaining detectors provide a direct electrical output. The main operational parameters of these typical detectors are given below in Table 1.
TABLE 1 ______________________________________ Detector Type scin./PMT gas Si--Li PIN-PD ______________________________________ Energy Range (kev) 5-1000 1-100 .1-100 1-100 Resolution @ 6 kev 1-2 kev 800 ev &lt;100 ev 1 kev Detection Efficiency 30% 50% &gt;90% &gt;70% Max. Count Rate 1000 200 1000 &gt;1000 (kHz) Cooling Require- none none LN.sub.2 (77K) &lt;25C ments ______________________________________
To provide a useful output, the detector must generate a signal that is significantly above thermal noise limits. An energy-sensitive x-ray detector provides a series of pulses that must then be quantified as to the energy of the x-ray each pulse represents. The energy resolution of this process determines the range and resolution of elements that may be effectively identified. The counting statistics of the pulses determine the statistics of the final quantitative measurement.
Conventional detectors are classified as avalanche and non-avalanche. Avalanche detectors feature some form of internal gain (such as the electron cascade in a gas proportional tube or photo-multiplier tube) to raise the signal level above background noise. Since they have no internal gain mechanism, non-avalanche detectors require extensive cooling such as by liquid nitrogen for reducing background noise. Another class of detectors includes charge-coupled array detectors (CCD's), typical examples thereof being commercially used in video cameras. A CCD detector can provide a non-avalanche effective gain equal to the ratio of the equivalent capacitance of the entire imaging area of the device (typically 10.sup.5 pF) to the capacitance of the output amplifier MOSFET gate (typically 0.25 pf).
Although normally utilized for the detection of infrared or visible light, the CCD has two basic characteristics that are advantageous for the detection of soft x-rays: A large active area (&gt;5 cm.sup.2 possible); and low inherent noise (less than 10 e- possible). The x-ray sensitivity of the CCD was first exploited extensively by James Janesick and others as a tool to characterize the performance of the CCD because it was observed that a good CCD in the x-ray domain was an excellent CCD in the visible domain. See, for example, Robinson et al., "Performance Tests of Large CCDs," Charge-Coupled Devices and Solid State Optical Sensors II (SPIE Vol. 1447, 1991), p. 214. In particular, testing in the x-ray domain led to identification of the important CCD characteristic "charge-transfer-efficiency" (CTE).
Conventional CCD's, as used in video cameras, have a number of drawbacks in x-ray detector applications, such as a parallel set of "shadow" registers that are used in the frame transfer process required for video format signals. These shadow registers are opaqued and not available for x-ray detection, wasting valuable detector area. Video CCD's operate at a high data rate such that low-noise operation is not possible, and thus are not optimized for low noise. Also, any window on the device package absorbs the low-energy x-rays of interest.
CCDs more suitable for x-ray detection are also known, being high-quality, scientific-grade, windowless devices that do not have shadow registers. Since the electrodes used for charge transport across the detector array block photon absorption, scientific-grade CCD's are often operated in a thinned, backside-illuminated mode. For these devices, the bulk silicon from the rear of the CCD chip is removed until the chip is only 10 microns thick. The chip is then mounted upside-down to allow maximum photon transmission and the resulting assembly is cooled to reduce thermal noise contributions.
A problem with the use of back-illuminated CCD's for x-ray detection, particularly at low energy levels, is that a significant proportion of the charge can recombine or be absorbed within the first few microns (farthest from the electrodes) of the substrate before effective trapping can occur. Attempts to avoid this problem include using implants to drive electrons toward the electrodes, the electrodes being all driven relatively negative during the integration time for avoiding excessive dark current accumulation (noise).
Although CCDs have been used in x-ray detection, it is believed that all prior art x-ray detection applications of the CCD are for intensity sensing, not energy sensing. Also, it is believed that all prior art applications of the CCD have utilized the device in an imaging mode.
Thus there is a need for an x-ray detector that has high energy resolution for detecting low-Z elements, high detection efficiency for rapid data acquisition, that is statistically accurate, that is inexpensive to provide, and that does not have excessively burdensome cooling requirements. There is a further need for a spectrometer that exhibits these advantages.